This invention relates to particle-in-gas filtration and particularly relates to changing ventilation filters in hazardous, toxic or radioactive environments. In such environments it is necessary to remove particulate matter from the atmosphere and to minimise redispersion of such particulate matter during the replacement of the filters. It is particularly important in nuclear installations that particulate radioactive material is removed from the atmosphere of the installation and is not redispersed so as to protect the operators of the plant from exposure to radioactivity.